Analyzing samples representative of downhole fluids is an important aspect of determining the quality and economic value of a hydrocarbon formation.
As the technology of oil and gas production advances, and environmental regulations become stricter, new demands are put on the industry to identify more cost-effective methods of reservoir control. A leading example of such control is the prediction, monitoring, preventing and removal of scale formation. A key request that directly related to the first three operations is in situ measurement of pH, together with the concentration of the critical ions, in aqueous borehole fluids. In particular, real time measurement of pH will offer valuable prediction of the initiation of nucleation that eventually leads to macroscopic scale formation. Also, in the ever-significant operation of in situ H2S detection, it is often a prerequisite that medium pH is known so that total inorganic sulfur can be deduced on the basis of thermodynamic equilibrium.
Present day operations obtain an analysis of downhole fluids usually through wireline logging using a formation tester such as the MDT™ tool of Schlumberger Oilfield Services. However, more recently, it was suggested to analyze downhole fluids either through sensors permanently or quasi-permanently installed in a wellbore or through one or more sensors mounted on the drillstring. The latter method, where successfully implemented, has the advantage of obtaining data while drilling, whereas the former installation could provide additional value as part of a control system for wellbores and hydrocarbon production therefrom.
To obtain an estimate of the composition of downhole fluids, the MDT tools uses an optical probe to estimate the amount of hydrocarbons in the samples collected from the formation. Other sensors use resistivity measurements to discern various components of the formations fluids.
General downhole measurement tools for oilfield applications are known as such. Examples of such tools are found in the U.S. Pat. Nos. 6,023,340; 5,517,024; and 5,351,532 or in the International Patent Application WO 99/00575.
Rapid and reliable pH measurement at downhole conditions, i.e., elevated temperature and pressure, and the presence of multiphase fluids, represents a formidable challenge to the existing techniques, such as potentiometric measurement of electromotive force based on glass electrodes, due to their poor stability and difficulty in interface renewal.
Although, their responses to changes in ionic composition are much faster than with potentiometric methods, conventional calorimetric methods using homogeneous reactions with indicator dyes often suffer from a lack of precision.
The measurement of pH is an art with an extended track record. The mainstream techniques are colorimetry and potentiometry, while most of those relevant to the present work involving entrapping dye molecules into thick polymeric films. In a review article, Cammann outlined the current scope and directions of future development for optical chemical sensors, in particular, for pH applications in: K. Cammann, “Optrode quo vadis?”, Sensors and Acutators B, 51, 1(1998), while Spichiger-Keller presented the fundamental principles of optical chemical sensors in: U. E. Spichiger-Keller, “Chemical sensors and biosensors for medical and biological applications”, Chapter 6(259-320) Wiley-VCH, Weinheim, 1998.
Apart from the entrapment into films, a variety of other methods based on either physical and chemical adsorption are used to immobilize the active (color changing) species. The known methods include sol-gel processes or bifunctional agents to bind the active species to a solid substrate.
Where optical transducers are applied to measure the response of the sensor to illumination, absorbance or transmission measurements can be used. In many cases, a fluorescence signals is monitored. Alternatively, it is known to use evanescent light or total internal reflection (TIR) measurements to detect a change in the optical properties of the active species. Another possible detection mechanism is based on surface plasmon resonance (SPR). Many of the above methods are used together with fiber optics to couple light into the system and connect light source, sensor and optical detector.
In modifying siliceous or metal oxide surfaces, one technique that has been used is derivatization with bifunctional silanes, i.e., silanes having a first functional group enabling covalent binding to the surface (often an Si-halogen or Si-alkoxy group, as in —SiCl3 or —Si(OCH3)3, respectively) and a second functional group that can impart the desired chemical and/or physical modifications to the surface. This process is generally referred to as silylation.
Silylation has been used for optical pH sensing purpose and is for example described by F. Baldini et al. in: F. Baldini and S. Bracci, “Optical-fibre sensors by silylation techniques”, Sensors and Actuators B, 11, 353(1993) using bromophenol blue as chromophore immobilized on controlled-pore glasses (CPG).
Calibration of pH sensors is a challenging issue. In one known approach a pH-independent wavelength is monitored while probing wavelengths where the entrapped dye indicator showed maximal transmittance. In addition to the wavelength where the variation of analyte's concentration is probed, another wavelength was probed, which is unaffected by the measurement but is subject to other intrinsic changes in the rest of the system, such as light source fluctuation and/or changes in optical fiber transmission mode. The latter changes are common at both wavelengths and can hence be eliminated. A pH sensor designed as such, using physically adsorbed methylene blue on the tip of an optical fibre, resulted in a rather wide pH range of 3-10 with a resolution of 0.015 units. An extra advantage of this “calibration-free” approach is the dampened temperature dependence of the resultant device, due to similar coefficient of the individual molar adsorptivities.
Though mature and effective in their own right, none of these aforementioned techniques is capable of direct applications to elevated temperature, high pressure and complex chemical compositions. The device that was able to operate in conditions most closely resemble those encountered in oilfield industry was an in situ pH sensor for hydrothermal fluids, as reported by Ding and co-worker in: K. Ding and W. E. Seyfried, Jr., “Direct pH measurement of NaCl-bearing fluid with an in situ sensor at 400° C. and 40 megapascals”, Science, 272, 1634(1996). Using a yttria-stablized zirconia membrane as the working electrode, they measured the potentiometric change as a result of pH variation under supercritical conditions. But this approach suffered from the slow response time (>20 minutes) and lack of stability, where the Ag/AgCl reference electrode was in direct contact with the fluids. Also, Gervais et al designed a pH sensor, using a fluorescein indicator, which was able to operate at pressure up to 250 MPa, as described in: M. Hayert, J-M Perrie-Cornet and P. Gervais, “A simple method for measuring the pH of acid solutions under high pressure”, J. Phys. Chem. A. 103, 1785(1999). The measurement of pH under wellbore conditions is made even more complex because of the pressure induced dissociation of weak acids. For example, neutral water undergoes a shift of about −0.73 pH/100 MPa.
More recently a method for pH measurement under downhole condition has been described in the published international patent application WO 2004/048969 A1. A colorant is added to a sample taken by a downhole monitoring system suspended into the wellbore from a wireline. The color change of the colorant is monitored by a suitable spectral analyzer and can be made indicative of, for example, the pH of the sample fluid.
Whilst there are numerous examples of optical pH sensors in other technical fields such as physiological application, the oilfield industry lacks simple and robust sensor to measure the pH under downhole conditions. It is therefore an object of the present invention to provide a sensor for pH measurement at high pressure and/or high temperatures. It is a further object of the present invention to provide downhole sensors and sensing methods for pH.